The presently disclosed embodiments relate generally to layers that are useful in imaging apparatus members and components, for use in electrostatographic, including digital, apparatuses. More particularly, the embodiments pertain to a method for forming an improved imaging member having a charge transport layer comprising a bottom layer and a top layer, wherein the layers have varying concentrations of a high quality hole transport material of a substituted biphenyl diamine, such as N,N,N′N′-tetra(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine to provide increased discharge rate. In addition, the discharge rate of the imaging member may be tuned by varying the thicknesses of the top and bottom layers of the charge transport layer. An imaging member using a benzimidazole perylene charge generating material having tunable electrical response characteristics is disclosed in U.S. Pat. No. 5,686,213, the disclosure of which is incorporated by reference herein in its entirety. In present embodiments, a particular configuration of the charge transport layer is used to provide an improved tunable imaging member. Incorporation of anti-oxidant materials into the charge transport layer helps reduce lateral charge migration (LCM).
As used herein, the discharge rate refers to the voltage drop over time and is based upon a discharge over a discharge interval at a given light intensity, wherein discharge is defined as the voltage drop or difference between the initial surface voltage before light exposure and the surface voltage after light exposure at the end of the discharge interval. Discharge interval is defined as the time period from the light exposure stage to the development stage (which is essentially the time available for the photoreceptor surface to discharge from an initial voltage to a development voltage) and light intensity is defined as the intensity of light used to generate discharge in the photoreceptor. The exposure light intensity influences the amount of discharge, and increasing or decreasing light intensity will respectively increase or decrease the voltage drop over a given discharge interval.
Electrophotographic imaging members, e.g., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment move under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors or imaging members have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL). Enhancement of charge transport across these layers provides better photoreceptor performance.
The demand for improved printing capabilities in xerographic reproduction is increasing, especially in achieving increased print speeds in xerographic machines. However, because an increase in print speed reduces the time available for the surface of the imaging member to discharge, any charge still in transit will result in a higher surface voltage on the imaging member during development and result in a negative impact on print quality. Commonly used high mobility molecules have been incorporated into the charge transport layer in an attempt to increase imaging member discharge rates. However, it was discovered that high mobility characteristics in these molecules did not necessarily impart high discharge rates. Thus, conventional formulations used to make these photoreceptor layers, while suitable for their intended purpose, do not resolve the print quality issues. However, changing the existing formulations to address such issues may impact the way the photoreceptor layers interact and could adversely affect other electrical properties.
The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.” The term “electrostatographic” includes “electrophotographic” and “xerographic.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.”